Is it not rotating at all, for everything, or just for somethings? If it is the latter, that is normal. Some apps and screens do not allow for rotating. They just aren't in the programming.
Now if it is happening all the time, no matter the screen or app, You could try turning the option off, restarting the phone, and then turning it back on to see if that resolves it.
Also, you should check for any updates for the phones firmware.
If none of this does fix it, you may have to try a "Factory Reset" or even a "Hard Reset". Both will wipe data though, the first not all, the second all. So they are really a last resort option.
To do a "Factory Reset" locate the pinhole on the device. It is usually on the edge, or near where the cards (SIM and SD) are inserted. Using something small, but not sharp, depress the button. It should restart the device.
NOTE: Doing this will clear 'all' the data on your tablet, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the tablet...
1. Make sure that the tablet is actually powered off.
2. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
3. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
4. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
5. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
6. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
7. Then press the "Power" button to hard reset the tablet.